Implement constrained variables

[blegat]

When using Mosek after a POI layer without any bridge in-between, the performance was catastrophic because the PSD variable where bridged to affine constraints as the bridges didn't see through the POI layers that PSD variables were supported.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 97.54%. Comparing base (2774d92) to head (e4c952b). :warning: Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   97.52%   97.54%   +0.02%     
==========================================
  Files           5        5              
  Lines        1452     1468      +16     
==========================================
+ Hits         1416     1432      +16     
  Misses         36       36              

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[joaquimg]

Nice! can you just add tests for constrained variable(s)?